[rrd-users] Transfer rate for rrd graph command

[Yu Watanabe]

Hello all.

I would like to ask a question regarding to the transfer rate for rrd graph command.
I was checking the following page to find out why the y-axis value seems to decrease
when the range of date changes.

http://oss.oetiker.ch/rrdtool-trac/wiki/RrdFaq

However, I couldn't quite understand the "transfer rate 120" meant in the paragraph.
Could someone explain following items?

1. What does value 120 means?
2. What does transfer rate 120 means?

Thank you 
Yu Watanabe